This invention relates to a process and apparatus for continuous single-stage production of a homogeneous solution of cellulose in water-containing amine oxides from cellulose and aqueous amine oxides, preferably N-methylmorpholine N-oxide (NMMO), at temperatures in the range from 50 to 130° C. under reduced pressure by water evaporation.
The dissolving of cellulose in amine oxides having a defined water content and at temperatures above 70° C. is known (DRP 713 486; U.S. Pat. No. 3,447,939). The dissolving proceeds very slowly. The dissolving takes place substantially more rapidly when the cellulose is dispersed in aqueous NMMO and the excess water is subsequently distilled off under reduced pressure while stirring at temperatures above 85° C. to simultaneously dissolve the cellulose (GB 8 216 566).
It is further known to mix comminuted cellulose with water-containing NMMO in an annular layer mixer (WO 96/33221) or a horizontal mixing chamber equipped with rotor and radial stirring elements (WO 94/28217). The homogeneity of the suspension can be improved by high consistency grinding. The suspension is converted into a solution by means of thin film evaporators (EP 356 419) or in the shearing zone of a horizontal screw dissolver (DE 4 441 468).
It is further known to produce cellulose solutions continuously by, in a first stage, the cellulose and aqueous NMMO being dispersed in a horizontal twin-screw kneader and the suspension being metered via an intervessel continuously into a horizontal single-screw kneader and converted into a solution under reduced pressure by water evaporation (A. Diener and G. Raouzeos Chemical Fibers International 49 [1999] 3 p. 40-42; DE 19 837 210).
The processes all have in common that the cellulose solution is fundamentally produced in two spatially separate stages, namely on the one hand in the suspending of the comminuted cellulose in the highly water-containing amine oxide with or without aftertreatment and secondly in the converting of the suspension into the actual solution by shearing and water evaporation.
It is an object of the present invention to provide a process and apparatus for continuous single-stage production of a homogeneous solution of cellulose in water-containing amine oxides. In other words, cellulose and water-containing amine oxide, preferably NMMO, are continuously converted in one stage into a homogeneous cellulose solution in an apparatus without explicit transportation and metering of a suspension and fed to the potential consumer along the shortest possible path.